Following the commercialization of small mobile devices such as laptops, smart phones and smart pads, wearable devices such as smart glasses, smart watches, smart rings, smart necklaces and smart bands are also being commercialized. In addition, miniaturized sensors and communication chips have become widespread, and thus the Internet of Things (IOT), in which various devices collect and communicate data, has arrived. However, the limited battery capacity of each electronic device is still limited, and particularly, battery management becomes an increasingly important issue as miniaturization and wear time of wearable devices increase.